Screen printable metallization pastes are used to make electrical contact to silicon photovoltaic (PV) cells and to connect PV cells together. The cells are connected in series by making electrical contact between the metallization layers and solder coated, Cu tabbing ribbons. Three primary paste formulations are used in silicon based PV cells: 1) rear side aluminum paste, 2) rear side silver tabbing paste, and 3) front side silver paste. FIG. 1 is a schematic drawing that shows an example of silver-based metallization paste 100. The paste 100 has silver particles 110, glass frit 120, and an organic binder 130 mixed with solvent. Examples of pure silver, rear tabbing pastes made by DuPont™ (Solamet® PV505) and by Heraeus (SOL230H) contain 45-66% conductive particles by weight and are designed to reduce silver loading and have a resulting thickness between 4 and 10 μm after screen printing, drying, and co-firing. In some examples, the silver particles are a mixture of silver flakes that are 1 to 3 μm in diameter and silver spheres with a distribution of sizes ranging in diameter from 100 nm to 5 μm. Examples of pure silver, front metallization pastes made by DuPont™ (Solamet® (PV17x) and Heraeus (SOL9235H) contain more than 90 wt % conductive particles. Such front metallization pastes contain spherical silver particles that range in size between 300 nm and 5 μm. Such pastes are formulated to print lines that have a high aspect ratio, compact densely to improve bulk conductivity, and make ohmic contact to the emitter layer of a silicon solar cell.
In the most widely commercialized Si solar cell architecture, aluminum paste is used on >90% of the back side to form an ohmic contact to p-type Si, creating a back-surface field to improve PV performance. Rear side silver tabbing paste occupies regions not coated with aluminum paste and is used to promote strong adhesion to Si as well as solderability to solder coated Cu tabbing ribbon, as soldering directly to the aluminum paste is challenging. Front side silver paste is formulated to penetrate through the anti-reflective coating to make ohmic contact to the front, n-type silicon. During silicon PV cell processing each paste layer is screen printed successively and dried at low temperatures (e.g., about 150° C.). Once the three layers are printed, the entire wafer is co-fired to over 750° C. for approximately one second in air to form ohmic contacts and promote paste adhesion to the silicon.
Silver based metallization pastes are inherently expensive because of the amount of Ag required and the commodity price of Ag, which is close to $625/kg in 2014. Silver based metallization pastes are the second largest materials cost for Si PV modules, and it is estimated that the PV industry currently uses more than 5% of all annual silver production. Continued, large increases in PV cell production will use more and more silver, which may become prohibitively expensive and unsustainable in the long-term.